(a) Field of the Invention
The present invention relates to an uplink channel estimation system for an orthogonal frequency division multiple access (OFDMA) system, and a method thereof.
(b) Description of the Related Art
In the OFDMA system, data are transferred on the uplink through sub-channels allocated to each subscriber. At this time, interference may occur due to a signal distortion caused by different channel environments and interference from the phase error of unmatched frequencies. When the signal distortion of channels and the frequency phase error are not compensated, the mutual orthogonality of subcarriers deteriorates to adversely affect other subcarriers. To solve this problem, a method of estimating channel variations from a reference signal and compensating for them is needed, as is a method of compensating for the frequency phase.
The channel estimation method includes a block-type estimation method using a preamble, a mid-amble, or a post-amble as the reference signal, and a comb-type estimation method using the pilot tone for a specific subcarrier as the reference signal to estimate the channel of the data subcarrier.
FIG. 1 shows an uplink sub-channel signal of the OFDMA system. Referring to FIG. 1, the first symbol of a sub-channel signal is comprised of a preamble, and the other symbols are comprised of data symbols. Each data symbol includes five pilot tones, among which four are variable-location pilot tones and the remaining one is a fixed pilot tone.
In this structure, the preamble is used to achieve channel estimation and channel compensation. However, the channel compensation is difficult to achieve in the channel environment in which the channel is estimated from the preamble, when the channel is a time-varying channel. Namely, the channel environment is different between a symbol subsequent to the preamble and the last symbol, in which case the channel compensation using a channel estimate obtained from the preamble is successful for the first symbol, but not for the last one.